Cosmological horizons, Noether charges and entropy Laura Donnaya;b, Gaston Giribetc a Center for the Fundamental Laws of Nature, Harvard University 17 Oxford Street, Cambridge, MA 02138, USA. b Black Hole Initiative, Harvard University 20 Garden Street, Cambridge, MA 02138, USA. c Physics Department, University of Buenos Aires and IFIBA-CONICET Ciudad Universitaria, Pabellón 1, Buenos Aires, 1428, Argentina. Abstract It has recently been shown that, in the vicinity of their event horizons, black holes exhibit an infinite-dimensional symmetry. This symmetry captures relevant physical information about the black hole, and in particular about its thermodynamics. Here, we show that the same holds in a cosmological setup. More precisely, we show that around the de Sitter cosmological horizon, an infinite set of diffeomorphisms preserving a sensible set of boundary conditions emerges. These boundary conditions are similar to those considered by Price and Thorne in the context of the membrane paradigm, and permit to accommodate interesting gravity solutions. As for other boundary conditions considered previously, they are preserved by an infinite-dimensional asymptotic isometry algebra that includes supertranslations and two copies of the Virasoro algebra. This symmetry has associated a set of Noether charges that turn out to be conserved, finite, and integrable. We derive these charges explicitly using the covariant formalism and arXiv:1903.09271v1 [hep-th] 21 Mar 2019 analyze their physical meaning by evaluating them to the case of de Sitter space. In this case, the zero-mode of the charge is found to account for the Gibbons-Hawking entropy of the cosmological horizon. We then consider a much more general set of solutions, including asymptotically de-Sitter black holes and asymptotically Taub-NUT-de Sitter black holes. 1 Introduction Observations indicate that we live in a dark energy dominated universe whose late time regime is well approximated by de Sitter space [1]. The accelerating fate of the universe implies the existence of a cosmological event horizon, a surface that surrounds any observer and that defines a point of no return: Distant comoving galaxies scape behind the cosmo- logical horizon and information from there can not be retrieved, as it happens with matter falling into a black hole. In fact, black hole horizons and cosmological horizons share many mathematical and physical properties. In particular, cosmological horizons are also thermo- dynamical objects; they have temperature and entropy [2]. As for black holes, the entropy of cosmological horizons obeys the area law1 A S = ; (1) 4G with A being the area of the cosmological horizon, set by the Hubble scale. This fact has profound implications: In particular, it means that quantum gravity in de Sitter space, if consistent at all, demands the product of the cosmological constant Λ and the Newton constant G to take special, non-continuous values [3]. It also raises the fundamental question about the actual reason for such special functional dependence with the area. In the last two decades, thanks to the holographic principle and its ramifications, we have gained insights about the meaning of the area law and of its generality. However, despite all these advances, a full understanding of it is still missing. The central question about the microscopic derivation of (1) has only been answered in idealized examples. In the case of black hole horizons, the question can be addressed in the context of string theory, but the situation is quite different with de Sitter space, for which an embedding in string theory has shown to be notably elusive [4]. Probably, the most important advances in understanding the thermodynamics of de Sitter space came from the dS/CFT correspondence [5]; but even in that context there are still open questions. Also in the case of black holes, which certainly offer a much more tractable scenario, there are important conceptual features of their thermodynamics yet to be understood. Such is the case of the information loss paradox, a longstanding problem that has recently been revisited. One of the recent attempts to address this problem is the idea of Hawking, Perry, and Strominger that the study of an intriguing infinite-dimensional symmetries –known as supertranslations– that black holes exhibit in their near-horizon region [6] could shed some light on the information hidden by the horizon (see also [7, 8] and references thereof). This study of infinite-dimensional symmetry is inspired by the recent application of similar ideas to the infrared physics in asymptotically flat space; see [9] and references therein. How much information is exactly encoded in this infinite-dimensional symmetry is still an open question. Nevertheless, the existence of such a large symmetry in the vicinity of event horizons is interesting on its own right, and the question remains as to what extent the thermodynamical properties of black holes can be captured by such a mathematical structure. In a more general context, it is interesting to investigate further the connection between the thermodynamical variables of event horizons, their infinite symmetries, and the associate 1We adopt the convention ~ = c = 1. 1 conserved charges. The aim of this paper is precisely to extend to cosmological horizons the study of infinite-dimensional symmetries which have recently been successfully applied to black hole event horizons, and then apply it to investigate their thermodynamics2. In order to do this in an efficient way, we will introduce a special set of asymptotic boundary conditions at the horizon, which are conveniently expressed in terms of Gaussian type coordinates. This will facilitate notably the task of finding the explicit change of coordinates needed to write the physically relevant solutions in the suitable near-horizon form. The discussion will be organized as follows: In Section II, we will review the main properties of de Sitter space and of its cosmological horizon. In Section III, we will derive the infinite-dimensional symmetry that emerges in the vicinity of the cosmological horizon and the Noether charges associated to it. In Section IV, we will apply these charges to de Sitter space and show that the zero-mode of the charge reproduces the Gibbons-Hawking entropy of the cosmological horizon. We will then extend the analysis to a much more general set of solutions: We will consider the case of asymptotically de Sitter and asymptotically Taub-NUT-de Sitter spaces, including black holes, for which we also study the infinite symmetries that appear both close to their black hole horizons and to their cosmological horizons. Section V contains our conclusions. 2 de Sitter space De Sitter space is the maximally symmetric solution to Einstein equations with positive cosmological constant. It can be defined by a hyperbola embedded in (4+1)-dimensional Minkowski space: On R4;1, we define the metric 2 2 2 2 2 2 ds5 = −dX0 + dX1 + dX2 + dX3 + dX4 ; (2) and impose the constraint 2 2 2 2 2 2 X1 + X2 + X3 + X4 − X0 = ` ; (3) where ` 2 R is the de Sitter radius. This defines a (3+1)-dimensional space with isometry group SO(4; 1), i.e. the remaining Lorentz part of the Poincaré isometry group of (2), after the constraint (3) has broken the affine part of it. The space obtained by these means is non-compact and has positive, constant curvature; namely 1 R = (g g − g g ) ; (4) µανβ `2 µν αβ µβ να which in particular implies the Einstein space condition Rµν = Λgµν with cosmological con- stant Λ = 3=`2. In the so-called flat slicing, the metric of de Sitter space takes the form p 2 2 2 Λ=3τ 2 2 2 ds = −dτ + e (dx1 + dx2 + dx3); (5) 2Infinite-dimensional symmetries in the context of cosmological models were also considered in [10–15]. 2 which follows from the embedding r^2 r^2 X = ` sinh(τ=`) + eτ=`;X = ` cosh(τ=`) − eτ=`; 0 2` 4 2` (6) τ=` τ=` τ=` X1 = e x1;X2 = e x2;X3 = e x3; 2 2 2 2 with r^ = x1 + x2 + x3. This coordinate system only covers the region X0 + X4 > 0 of the geometry. In these coordinates, de Sitter space has a clear cosmological interpretation: It describes an exponentially expanding isotropic, homogeneous universe with Hubble constant H ≡ `−1. The space presents a cosmological horizon at the characteristic distance H−1. The causal structure of the space, and in particular the existence of the horizon, is clearly captured by the static coordinate system, which is defined by p p X = `2 − r^2 sinh(t=`);X = `2 − r^2 cosh(t=`); 0 4 (7) X1 =rζ ^ 1;X2 =rζ ^ 2;X3 =rζ ^ 3; 2 2 2 with ζ1 + ζ2 + ζ3 = 1. In these coordinates the metric reads 2 2 2 2 2 2 −1 2 2 A B ds = (H r^ − 1)dt + (1 − H r^ ) dr^ +r ^ γABdz dz ; (8) 1 2 where γAB is the metric of the unit 2-sphere (A; B = 1; 2); ζi = ζi(z ; z ) with i = 1; 2; 3 give a standard embedding of S2. We clearly observe from (8) that the event cosmological horizon appears at the constant radius r^ = H−1. The static patch corresponds to the region 2 −2 r^ ≤ H ; see Figure 1. In this region, the Killing vector @t is timelike. The time-dependent picture is recovered in the region r^2 > H−2; in the large r^ limit this is given by r^ ' eτ=` + ::: where the ellipsis stand for terms that are subleading in the large τ limit. 3 Infinite-dimensional symmetry 3.1 Horizon boundary conditions Now, let us analyze the region near the cosmological horizon and derive the infinite- dimensional symmetries that appear there. We can study this by prescribing a set of boundary conditions that employs Gaussian type coordinates similar to those considered in the context of the membrane paradigm [16].